In recent years, lithium secondary cells are widely used as a power source for electronic equipment such as portable communication equipment and a notebook-sized personal computer. In addition, requests for resource and energy savings are raised for the protection of the earth's environment on an international scale. The lithium secondary cell is being developed as a motor driving battery for an electric vehicle and a hybrid electric vehicle (hereinafter also referred to simply as “electric vehicle etc.”).
A lithium secondary cell typically comprises an internal electrode body including a hollow cylindrical winding core, a positive electrode plate and a negative electrode plate wound around an external peripheral wall of the hollow cylindrical winding core with a separator disposed therebetween, a nonaqueous electrolyte solution impregnating the inside of the internal electrode body; a cylindrical cell case being opened at both ends for housing the internal electrode body; and two electrode caps sealing the above-described internal electrode body at both ends of the cell case.
Among them, at least one of the electrode caps must be provided with a current lead-out function for leading current from the internal electrode body to the outside of the cell; a pressure release function for preventing explosion of the cell at times when pressure inside of the cell increases abnormally; and a function as an electrolyte solution injection port at the time when electrolyte solution is injected into the internal electrode body contained inside the cell case. While each of the electrode caps is a small volume part among parts constituting a lithium secondary cell, as shown in FIG. 1, the electrode caps are parts greatly influencing current output as well as the endurance of the cell.
Conventionally, an electrode cap is, as shown in FIG. 17, constructed by combining parts such as an electrode pole 50, a bolt 44, a nut 43, a cap 47, a metal ring 42, a pressure release valve 49, an electrolyte solution injection port 48, a ceramic washer 45, a backup ring 46, etc. (See JP-A-9-92335).
However, the electrode cap described in JP-A-9-92335 is provided with a current lead-out function, a pressure release function and a function as an electrolyte solution injection port, but it is constructed by a number of parts, thus giving rise to a problem that assembly efficiency on a cell is bad and mechanical production is difficult.
In addition, this conventional electrode cap has, as shown in FIG. 17, a pressure release valve 49 disposed in the vicinity of the outer peripheral portion of the electrode cap remote from the central axis of the cell case 54 and thus is not good at releasing gas from the hollow portion of the winding core in which a lot of gas causing an increase of inner pressure is contained, giving rise to a problem that explosion of a cell cannot be prevented unless pressure release valves are disposed at both the electrode caps of the positive electrode and the negative electrode.
Moreover, the conventional electrode cap has, as shown in FIG. 17, electrolyte solution injection port 48 disposed in the vicinity of the outer peripheral portion of the electrode cap remote from the central axis of the cell case 54 like the pressure release valve 49, and thus the hollow portion of the winding core cannot be impregnated from a lower portion of the cell with the electrolyte solution through an injection nozzle, but is impregnated with the electrolyte solution by poring from an upper side of the internal electrode body, giving rise to a problem that impregnation of the electrolyte solution is not easy and injection of the electrolyte solution takes a fair amount of time. In addition, the top or bottom electrode cap has the pressure release valve 49 and the electrolyte solution injection port 48 disposed separately, and thus an area of a portion requiring sealing gets larger, giving rise to a problem that the electrolyte solution is apt to leak.
On the other hand, in order to drive a motor, a plurality of cells are brought into connection in series to secure a voltage necessary for driving. Actually, in electric vehicles etc., currents of 100 V or more could frequently flow by chance. Accordingly, in order to realize a high output feature and a large current feature, it is important to adopt a structure that will reduce connection resistance due to connection of cells as much as possible.